How would a detective in John Yates's police force deal with Blanche and Stanley from A Streetcar Named Desire, if Blanche had called them in? At the end of the play, Stanley rapes Blanche, his sister-in-law, and has her committed to an asylum.

In an interview with the Guardian today, John Yates suggests his officers would not acquit themselves very well. He says Metropolitan police do not apply the same professionalism to a rape allegation as other alleged crimes. He admits there would be a fair chance his force would not take Blanche's allegation seriously.

Cases of "stranger rape" are easier to substantiate – CCTV cameras can prove an ambush, injuries and marks prove rough treatment, and blood, semen and saliva are all incriminating.

But things get trickier with rape by an acquaintance. Advances in DNA testing mean suspects rarely rely on the argument that sex didn't take place, and so instead the issue turns on consent.

A relaxation of social attitudes has seen women become less reticent to report being raped by an acquaintance – a Blanche being raped by a Stanley. John Yates said in 85% of reported rapes the accused was known to the victim.

As complaints of this type have increased, conviction rates have plummeted, to 5.7% last year - the lowest in Europe. In 2005 the rate dropped as low as 5.4%. But in the 1980s it was 33% - one in three rape allegations ending in conviction. Now it is one in 20.

One reason for the fall is public cynicism. A poll by Amnesty International in 2005 found one in three respondents believed a woman was partly or wholly responsible for being raped if she acted flirtatiously, while 26% held the same view if she was wearing sexy or revealing clothing.

Another reason is alcohol. Increased alcohol consumption by young women in the last 20 years has made it harder for juries to convict. They are reluctant to convict in cases where a woman was drunk.

Public figures have been quick to decry the rape conviction rate – David Cameron called it a moral failure. The director of public prosecutions, Sir Ken Macdonald, said: "I am determined to improve the way we deal with these cases from start to finish. We must make sure that we do what we are supposed to do when handling these cases."

But there has been confusion in the government. In autumn last year it abandoned its proposal to use expert witnesses to brief juries on the "myths" surrounding rape. This would have seen experts explain to juries that rape victims will often not report the rape immediately, and also that they may appear controlled rather than emotional when explaining their story. This was dropped because judges warned the plan could lead to miscarriages of justice.

Sarah Campbell of the Fawcett Society is not wholly dismissive of the government's efforts, welcoming initiatives such as allowing victims to give evidence by videolink, but warns of a lack of funding for rape crisis centres. "In 1984 there were 80 rape crisis centres, now there are around 35," she said.

In his interview today, John Yates backed a programme of myth debunking. "We've got to get a better understanding of how victims react: don't expect victims to report right away, and don't expect victims to scream and shout."

While it is unclear what reforms the government will implement in the courts, Yates is urging improvements at the earlier investigation stage, where he says the attrition rate – cases collapsing – is greatest.